It is generally known among the bicyclists that the head tube, the handlebar sleeve and the front fork of a bicycle are vulnerable to becoming loose, and that they are susceptible to making an aggravating noise when the bicycle is in operation. A remedial measure of the prior art is illustrated in FIG. 1, in which a front fork 71A is shown to be provided on the inner wall of the upper end thereof with a threaded portion 72A to which a rod 40A and a front tube 60A are fastened by means of an adjustment nut 21A, a compression ring 50A, and a seat ring 61A. Such a prior art fastening method as described above is defective in design in that the front fork 71A must be provided with the threaded portion 72A which cannot be machined easily and economically.
Another prior art remedial measure is shown in FIG. 2 and is provided with two spring leaves 1B, which are located in the front fork 71B by means of a fastening bolt 11B such that the two spring leaves 1B are retained securely in the front fork 71B when the two spring leaves 1B are pulled upwards. In addition, the remedial measure is provided with a restraining ring 50B, which can be so pressed as to cause the handlebar sleeve 40B to be fastened securely with the front fork 71B. The fastening effect of the two spring leaves 1B is undermined by the fact that the front fork 71B is made of a steel material and is therefore incompatible with the two spring leaves 1B which work effectively with a tube wall of a soft and nonmetallic material. In addition, such a prior art remedial measure is unsafe in view of the fact that the handlebar sleeve 40B and the front fork 71B are rather vulnerable to becoming disengaged when two screws 41B become unfastened accidentally. Moreover, the front fork 71B is provided with a bearing seat which is fastened with the bottom of the front fork 71B by an external force and is therefore not cost-effective.